Regcensus

Fixes documents endpoint to `/values/documents`

Adjusts documentType IDs to adjust for recent rebasing.

Adds ability to use both documentType and seriesID to filter get_periods values.

Adds unit testing through `pytest`.

Adds a download feature for `get_values`.

Updates some functions to reflect upcoming changes to the data (mostly agency-level data).

Updates documentation to reflect changes and for some clarifications.

Documentation updates, values error catching, and small fixes to list functions

Adds a documentType parameter option to get_documents() and adds some more documentation to README.